Unsaturated polyester resins are well-known and useful materials for making molding compositions with a broad application in the manufacture of automotive and other products. One of the main deficiencies of cured unsaturated polyesters and fiber reinforced polyesters (FRP's) made from then is the inherent brittleness of the polymer matrix. Cracking at room and elevated temperatures is a principal cause of failure in production and in service of the molded parts.
A number of strategies for improving the toughness and crack resistance of polymeric materials have been demonstrated in thermoset epoxies. The most common and successful technique is the creation of a second elastomeric phase through a phase separation process during cure. This process has found only limited success in unsaturated polyesters for several reasons. The reactive liquid rubbers used to make the elastomer phase contribute to an undesirable high viscosity, the rubber phase separates before curing, and the reactive liquid rubbers contribute to higher shrinkage resulting in poor surface quality of the molded part.
U.S. Pat. No. 4,530,962 discloses a method of reacting an epoxy resin with a low molecular weight elastomer having reactive end groups. This helps solve the phase separation problems with elastomers because the epoxy groups help solubilize the elastomer in unsaturated polyester resins.
U.S. Pat. No. 4,290,939 discloses a second approach to incorporating toughening unsaturated polyesters. A reactive liquid polymer is coreacted with the precursors to an unsaturated polyester resin. Reactive groups on the ends of the reactive liquid polymers are coreacted into the unsaturated polyester chains, thus increasing the solubility of the reactive liquid polymers in the resin compositions. This process creates an unsaturated polyester resin of diverse composition that is subsequently used as a sole polyester and which is crosslinked with an ethylenically unsaturated monomer.
The method of U.S. Pat. No. 4,290,939 forces the reactive liquid polymer to undergo the high temperature condensation cooking process used to make the unsaturated polyester. The polymers formed by this process potentially have one or more reactive liquid polymers incorporated randomly in their backbone. No control over the microstructure of the unsaturated polyester is achieved other than controlling the number of residual carboxyls or hydroxyls which determine the molecular weight of the polymers.
European Patent Application 87301544.0 discloses unsaturated polyester compositions containing at least one block copolymer. The block copolymer includes both polyester parts and liquid rubber compound parts as constituent segments. When pre-made unsaturated polyester blocks were linked to liquid rubber blocks in production examples 4 and 8 using difunctional isocyanates in a one step process, they obtained, in addition to the desired block copolymers, "crosslinked" by-products of unsaturated polyesters and polybutadiene glycols.